#include<iostream>
#include<iomanip>
#include<cmath>
using namespace std;
int main()
{
	int n, m;
	double p, q;
	while (cin >> n >> m) {
		if (!n && !m) {
			break;
		}
		cin >> p >> q;
		double rate;
		/*if (n == 0) {
			cout << 0.00 << endl;
			continue;
		}
		if (m == 0) {
			cout << 1.00 << endl;
			continue;
		}
		if (p - 1.0 < 1e-6 || q - 0.0 < 1e-6) {
			cout << 1.00 << endl;
			continue;
		}
		if (p - 0.0 < 1e-6 || q - 1.0 < 1e-6) {
			cout << 0.00 << endl;
			continue;
		}*/
		if (n == 0) { printf("0.00\n"); continue; }
		if (m == 0) { printf("1.00\n"); continue; }
		if (p == 0 || q == 1) { printf("0.00\n"); continue; }
		if (p == 1 || q == 0) {
			printf("1.00\n"); continue;
			if (p == q) {
				rate = (1.0 * n) / (m + n);
			}
			else {
				double k = q * (1 - p) / (p * (1 - q));
				rate = (1 - pow(k, n)) / (1 - pow(k, m + n));
			}
			cout << fixed << setprecision(2) << rate << endl;
		}
		return 0;
	}
}